This invention relates generally to nuclear reactors and more particularly, to a method for reducing the usage factor of the feedwater nozzle in a nuclear reactor.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A core assembly is contained within the RPV and includes the core support plate, fuel bundles, control rod blades and a top guide. A core shroud typically surrounds the core assembly and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
Internal structures of operating BWRs are susceptible to various corrosive and cracking processes. Stress corrosion cracking (SCC) is one known phenomenon occurring in reactor components, such as structural members, piping, control rod guide tubes, fasteners, and welds, exposed to high temperature water. The reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to SCC.
Most BWR pressure vessels include at least one feedwater nozzle which connect feedwater spargers with the feedwater supply. Feedwater nozzles usually include a thermal sleeve and a safe end which couples to the feedwater supply line. Feedwater is distributed through the spargers that deliver the flow of water to the reactor core to help maintain proper reactor water level. Feedwater nozzle cracking can be caused by thermal fatigue. The thermal fatigue can be caused by a high cycling mechanism which involves rapid temperature cycling and usually initiates the cracking. Also, a low cycle mechanism which is due to changes in the feedwater or reactor water flow temperature can cause the formed cracks to propagate.
Rapid temperature cycling can be caused by leakage flow passing the thermal sleeve and safe end seals. This leakage flow mixes in a turbulent manner with hot downcomer flow in the annulus between the nozzle and the thermal sleeve. The fluid impinges on the nozzle wall before cold and hot water mixes completely and exposes the metal surface to cold and hot water alternatingly, causing thermal cycling of the metal surface. This metal temperature cycling can have a magnitude of up to 50 percent of the difference in temperature between the feedwater and the downcomer water. The metal temperature cycling can have frequencies of between 0.1 and 1.0 Hz and can rapidly initiate cracking. Rapid temperature cycling can also be caused, in the absence of leakage flow, by turbulent downcommer flow causing the thermal boundary layer around the cold thermal sleeve to be broken up intermittently, exposing the nozzle to the alternating cold and hot water streams. Incompletely mixed sparger discharge flow and downcommer fluid carried back to the nozzle also causes some rapid cycling.